Among prior small-sized optical systems using a catadioptric optical system to obtain full 360°-direction (full-panoramic) images, there is one made up of a front unit comprising a transparent medium that is rotationally symmetric about a center axis and includes two internal reflecting surfaces and two transmitting surfaces (an entrance and an exit surface) and a rear unit that is rotationally symmetric about the center axis and has positive power, as set forth in patent publications 1 and 2.
Patent Publication 1
U.S. Pat. No. 4,566,763
Patent Publication 2
U.S. Pat. No. 5,473,474
A problem with either of patent publications 1 and 2 is, however, that images go worse because an entrance pupil is not positioned near the entrance surface and the effective diameter of the entrance surface grows large, so that there is much harmful flare light coming from the zenithal direction or the ground. Another problem is that aberrations are not always well corrected, nor is resolving power high.
Such being the case, a main object of the invention is to provide an optical system that is adapted to obtain an image having a full 360°-direction angle of view or project an image at a full 360°-direction (full-panoramic) angle of view, and is of small-format size and well corrected for aberrations without being affected by flare light yet with high resolving power.